Ductile cast iron pipe having constricted end casing

ABSTRACT

A pipe or similar tubular member of ductile cast iron having an end casing. The thin walled casing has a curved profile at its entrance divided into two sections, of which one is formed by centrifugal casting, whereas the other, of lesser curvature, is made by hot or cold forming with a simultaneous axial compression and a radial bending directed towards the axis of the pipe. The invention is useful for forming fluid tight seals in pipelines using radial compression packings.

The present invention relates to pipes or tubular members made ofductile cast iron or spheroidal graphite (nodular) cast iron obtained bycentrifugal casting and provided with an end casing having a smoothinternal surface and a constriction at its entrance.

Casings of this type in which the constriction is in the form of aninturned collar or a restriction are known for steel, aluminium andplastic pipes. The inturned collar or restriction can be made by hot orcold deformation. The collar can equally well be obtained by molding andthen placed in the end of the casing and fixed rigidly thereto, thecasing not being deformed. When formed by deformation, the casing andcollar are of unitary construction. When the collar is moldedseparately, the casing is in two parts.

Such a collar or end restriction for a casing defines in the casing arecess for receiving a fluid-tight packing of simple form, for exampletorroidal or prismatic, in an assembly of pipes, and prevents thispacking from being expelled in the event of an over pressure of thefluid contained in the pipe. It can also help, in the case of plasticpipes, to make the casing more rigid and therefore conserve its shapewhich facilitates assembly.

Hitherto, the constrictions at the entrance to casings of ductile castiron pipes have been made directly by centrifugal casting using a corehaving a special shape which creates an internal circular shoulder atthe entrance of the casing. However, for large scale manufacture,certain requirements, particularly dimensional tolerances, make itnecessary to make the wall of the casing much thicker than that of themain portion or barrel of the pipe.

The object of the invention is to provide a pipe of ductile cast ironcomprising a casing which is lighter and easier to make, that is acasing having a thin and smooth wall for which the entrance collar isobtained by deformation. This problem is difficult to resolve becauseductile cast iron, although deformable, has elongation characteristicssubstantially inferior to those of steel or aluminium, or even to aplastic material such as rigid polyvinylchloride. There is therefore ahigh risk, if one is seeking to employ a classical casing for fluid typepackings of simple form, for example torroidal or prismatic, thatrupturing or tearing will occur on the external surface of the casing inthe zone of maximum flexing of the thin wall of the casing.

Accordingly, the present invention provides a pipe or similar tubularmember of ductile cast iron having an end casing with a constriction atits entrance, said casing having a wall thickness which is substantiallyconstant and substantially equal to that of the main barrel portion ofthe pipe or tubular member, said casing having at its entrance a medianprofile in the form of two successive curved sections, one said sectionadjacent the entrance of the casing having a curvature substantiallyless than that of the other said section.

The invention also has as its object a method for making such a pipe ortubular member of spheroidal graphite cast iron and provided with an endcasing. This method is characterized in that a pipe having a preformedcasing is formed by centrifugal casting using a end casing shell and acorresponding casing mandrel with an annular space provided between themhaving, from the entrance of the casing, a first convergent rectilinearprofile followed by a second convexly curved profile, and then, afterremoval of the centrifuged pipe from the mold, the part having arectilinear profile is inwardly turned towards the axis of the pipe toprogressively form a restriction by exerting on the end of the preformedpart a force which comprises an axial component and a radial componentdirected towards the axis of the pipe.

This form of casing and this method of manufacture allow, surprisingly,pipes of spheroidal graphite cast iron to be obtained economically whichhave a thin walled casing of unitary construction not susceptible totearing and in which the spheroidal graphite structure is preserved.

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a half view in median section of a ductile cast iron pipe witha casing in accordance with an embodiment of the invention;

FIG. 2 is a partial half view in median section of a preformed casingobtained by centrifugal casting;

FIG. 3 is a diagrammatic half view showing the bending into shape of themolded preform of FIG. 2;

FIG. 4 is a half view in section of a fluid type assembly between themale end of a pipe and the casing of the pipe of FIG. 1;

FIGS. 5 and 6 are microphotographs enlarged three hundred times showingthe structure of the casing respectively at the exterior and interioredge of the casing zone where the flexing constraints are at a maximum;

FIG. 7 is a microphotograph at the same enlargement of a point situatedat the exterior edge of the entrance of the casing in a region of thecasing which is deformed but not situated in the zone of maximumflexing; and

FIG. 8 is a microphotograph at the same enlargement showing thestructure at a point at the exterior edge of the non-deformed part ofthe casing.

The pipe 1 of spheroidal graphite or ductile cast iron shown in FIG. 1,cast by centrifugation about an axis XX, comprises a barrel portion 2 ofsubstantial length, which for convenience of illustration is broken intwo and of which only a small part is shown, and a casing 3 connected tothe barrel portion by rounded portions 4 of small radius of curvatureand opposed concavities.

The casing 3, which has at all points substantially the same wallthickness e as the barrel portion 2, has an internal surface 5 and anexternal surface 6 which are completely smooth and free of sharp edges.This casing comprises several successive parts which will be describedhereafter from the base of the casing towards it's entrance.

At the base of the casing, a truncated conical part 7, which divergestowards the exterior of the casing at a relatively small angle, isconnected to the rounded portions 4. An entrance recess 9 of the casingfor receiving a fluid-tight packing is connected to this part 7 by awide concave rounded portion 8.

The recess 9 is itself composed of two successive parts having a curvedconvex profile. A part 10 extends from the rounded portion 8 to thecircle 11 of greatest diameter of the casing and of which the radius ofcurvature r is moderate. This radius r is a multiple of the thickness e,for example, in the order of three times e. A part 12 extends from thecircle 11 to the entrance of the casing and is connected tangentially tothe part 10. The radius of curvature R of this part 12 is distinctlygreater than that of the radius of curvature r defined above.

The end face 13 of the part 12 is frusto-conical, converging towards thebase of the casing and approximately perpendicular to the free ends ofthe surfaces 5 and 6. This face 13 terminates, at end of least diameter,in a short cylinder 14 lying along the axis XX and of which the diameterD is substantially less than the maximum diameter of the casing but atleast equal to the smallest diameter d of the frusto-conical section 7.

The two parts 10 and 12 of the casing entrance form thus anunsymmetrical profile having two curves, one being relatively large forthe interior part 10, and the other being small for the exterior part12. The profile of the two parts 10 and 12, that is to say of the casing3 in the neighborhood of its entrance, can be compared to that of anaerodynamic deflector for an automobile.

In order to obtain the casing described above, one proceeds in thefollowing manner (FIGS. 2 and 3):

A pipe 1^(a) having an end providing a casing preform 3^(a) is formed bycentrifugal casting. After removing from the cast, this preform is givenits final shape by simultaneous axial compression and bending by meansof a die having at least in part the desired shape.

According to the embodiments shown in FIG. 2, the pipe 1 is obtainedfrom a metal centrifugation mold or shell 15 having at its casing end ahollow impression which defines exactly the external profile of therounded portion 4, the tapered section 7, the rounded portions 8 and thepart 10 of the pipe 1. However, slightly before reaching the circle 11,the impression in the cell is in the form of a frusto-cone 16 whichdiverges towards its free end approximately according to the same angleof taper as the tapered section 7, which gives to the casing preform3^(a) the shape of a very elongated S.

At this end of the shell is fixed, in a known manner and not shown, ahollow casing mandrel 17 rigid in rotation with the shell 15. Theprofile of the external surface of the mandrel 17 corresponds to that ofthe internal shape of the shell and forms with the latter an annularspace 18 of which the width, measured in an approximately radialdirection, is constant and corresponds to the thickness e of the pipe 1to be obtained. From the base towards the entrance of the casing, thehollow mandrel 17 has thus a frusto-conical end or nose 19 with arounded extremity which corresponds to the tapered section 7 of thepipe, a concave connecting section 20 which corresponds to the roundedportion 8, a second convex connecting section 21 which corresponds tothe part 10, a frusto-conical tapered section 22 corresponding to thesecond frusto-conical portion 16 of the shell, and a radial externalflange 23 for fixing against the shell providing the end face 13^(a) ofthe preform 13 which is internally bevelled at 14^(a).

By centrifugation, a pipe 1^(a) with a casing preform 3^(a) is cast inthe shell 15 provided with the mandrel 17. The pipe 1^(a) is thenremoved from the mold and the preform 3^(a) (FIGS. 2 & 3) has at itsentrance a frusto-conical taper 12^(a) followed by a curved section 10,a rounded connecting portion 8 and a tapered section 7.

After cooling, the pipe 1 is placed in a press, for example,horizontally (but it could be vertically) of which the frame is notshown. This press comprises a fixed plate 24 against which the end faceof the male end of the barrel 2 abuts, and a die 25 fixed on a jack notshown (of the screw or fluid pressure type) placed near the end face ofthe casing preform 3^(a) (FIG. 3). The die 25 has a widely taperingfrusto-conical entrance 26 having a entrance diameter greater than theexternal diameter of the end face of the preform 3^(a). This entranceconverges to its connection, with an appropriate transition, with aconcave curved profile wall 27 having a decreasing diameter, of whichthe curvature is equal to that of the second part 10 of the casing 3 tobe obtained.

With the aid of this press, by actuating the die jack 25, the jack canbe slowly moved along the axis X--X in the direction of the arrow f, theplate 24 being fixed. A progressive axial compression and bending actionis thus exerted simultaneously on the preform 3^(a). Under this action,the preform evolves progressively towards the final shape of the casingin the following manner.

The die 25 engages its entrance 26 on the end face of the preform 3^(a)(FIG. 3: position A in full lines) without initially deforming it. Then,progressively, the die exerts on the end face 13 an axial force which isaccompanied by a bending force towards the axis X--X. This progressivebending force is slow and impresses an increasing curvature on therectilinear generatrices of the frusto-conical surface 12^(a) (positionB in chain dotted lines). In continuing its advance, the die 25 causesthe end of the preform 3^(a) to penetrate the curved profile cavity 27.The axial travel continues thereby to exert a peripheral bending forceon the whole of the circumference of the preform, of which thefrusto-conical tapered section 12^(a) finishes by following completelythe curvature of the cavity 27 (position C in chain dotted lines). Atthis moment, the die 25 is stopped by controlling the driving jack. Thepipe 1 of FIG. 1 is therefore finished.

It will be noted that the first curved section 10 cast from the casinghas undergone no deformation except in the immediate vicinity of thecircle 11. Furthermore, the maximum bending force has been exerted in acircular zone situated at the connection of the first curved section ofthe preform 3^(a) and of the casing 3 with the second initiallyfrust-conical part 12^(a) of the preform 3^(a), now become a curvefollowing the shape 12. No tearing or splitting has occurred in thisregion of maximum bending which is situated at the circle of maximumdiameter of the casing. This good result is attributed to the fact thatthe radial bending force exerted by the die 25 is continuouslyaccompanied by an axial compression force in the direction of the arrowf.

Microphotographs enlarged three hundred times have been taken at theexterior edge (point M in FIG. 5) and the interior edge (point N in FIG.6) of the region of maximum bending. These microphotographs show that nofissures have occurred at these points, nor indeed tearing in the walls,and that the graphite remains spheroidal.

Another microphotograph (FIG. 7) has been taken at a point P situated atthe exterior edge of the entrance of the casing 3 which has undergone adeformation of greater amplitude than at the points M and N but has beensubjected to a smaller bending force. The graphite here also hasremained spheroidal.

Finally, a microphotograph (FIG. 8) has been taken at a point Q situatedat the exterior edge of the non-deformed part 10. This shows thespheroidal graphite structure obtained by centrifugal casting and servesas a comparison reference for other microphotographs.

The assembly of microphotographs of FIGS. 5 to 8 shows that thestructure of the pipe, which was originally spheroidal graphite, has notsuffered during cold deformation in the press 24-25.

FIG. 4 shows a fluid-tight assembly between two pipes of ductile castiron such as the pipe 1. This assembly is of known type having a radialcompression packing.

A free torroidal fluid-tight packing 28 is arranged in the recess 9 ofthe entrance of the casing 3 of one of the two pipes and bearselastically against the curved part 10 formed by molding and the curvedpart 12 formed in the press. The male end 29 of the other typepenetrates the packing 28, passing through the cylindrical entrance withlittle play, and extends within the interior of the tapered surface 7until it abuts against the base of the casing (interior part of thesurface 7 or rounded portion 4). The packing 28 is squashed radiallybetween the male end 29 and the casing 3 and is practically imprisonedin this position owing to the fact that it abuts against the wall 10 andthe small annular space remaining free between the entrance of thecasing and the external surface of the male end 29.

In the example above, the press forming of the casing 3 has been carriedout on the preform 3^(a) in the cold. However, this forming can alsotake place hot in the same press with the aid of an appropriate sleeve,the frusto-conical part 12^(a) of the preform 3^(a), the neighboringpart 10, not intended to be deformed, remaining cold.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A pipe or similartubular member, having a main barrel portion, said pipe being comprisedof spheroidal graphite ductile cast iron having an end casing with aconstriction at its entrance, said casing having a wall thickness whichis substantially constant and substantially equal to that of the mainbarrel portion of the pipe or tubular member, said casing having at itsentrance a median profile in the form of two successive curved sections,one said section adjacent the entrance of the casing having a curvaturesubstantially less than that of the other said section, wherein saidgraphite is spheroidal throughout the entire member.
 2. A pipe orsimilar tubular member according to claim 1, wherein an end face of thecasing is perpendicular to internal and external surfaces of saidcasing.
 3. A pipe or similar tubular member according to claim 2,wherein said end face of the casing meets, at its internal edge, acylindrical surface forming part of the casing entrance.
 4. A pipe orsimilar tubular member according to claim 1, wherein the section ofgreater curvature is connected by a concave rounded portion to afrusto-conical section which converges towards the base of the casing.5. A pipe or similar tubular member according to claim 4, wherein thefrusto-conical section is connected to the main barrel portion of thepipe by a double rounded portion.